1. Field of the Invention
The present invention relates to an antenna device, a card-type information medium, and a communication terminal apparatus.
2. Description of the Related Art
Some recent communication terminal apparatuses, such as cellular phones use RFID (Radio Frequency Identification) systems, so that the communication terminal apparatuses can be used as reader-writers or RFID tags. In a system that uses an HF communication band, the communication range between antenna devices depends on magnetic flux passing through coil antennas. That is, to provide a substantial communication range between antenna devices, the sizes of their coil antennas need to be large. However, increasing the sizes of coil antennas hinders the size reduction of communication terminal apparatuses. Thus, a technique of providing an RFID function to a communication terminal apparatus, such as a cellular phone, has been proposed in which a predetermined communication range is provided with a coil antenna occupying a reduced area.
An invention disclosed in International Publication No. WO 2012/033031 relates to an antenna device providing a predetermined communication range with a coil antenna thereof occupying a reduced area.
FIG. 14 is a sectional view of the antenna device disclosed in International Publication No. WO 2012/033031. In FIG. 14, an antenna device 211 includes an antenna coil 100 including a coil conductor 22 wound around an insulator core 21, and two planar conductors 11A and 11B each including a surface extending along the winding axis of the coil conductor 22, with an edge portion of each of the planar conductors 11A and 11B being in proximity (adjacent) to a corresponding one of two coil openings of the coil conductor 22. The two planar conductors 11A and 11B function as booster antennas for the antenna coil 100 by an effect of magnetic-field coupling to the antenna coil 100. Thus, despite the small size of the antenna coil, the antenna device has electrical characteristics that are substantially the same as or superior to electrical characteristics of a large-sized antenna coil.
In the configuration disclosed in International Publication No. WO 2012/033031, since the two planar conductors are positioned in proximity to each other, the two planar conductors are electromagnetically coupled to each other, such that induced currents generated in the respective planar conductors cancel each other out. Therefore, the functions of the two planar conductors as booster antennas are deteriorated, and the allowable communication range is thus narrowed.
If the two planar conductors are not in proximity to each other, a wide air gap is provided between the planar conductors. Therefore, magnetic flux from a major surface of one of the planar conductors runs through the air gap and enters the other major surface of that planar conductor, forming a small loop. Such magnetic flux forming a small loop provides substantially no contribution to the communication between antenna devices. Therefore, the functions of the booster antennas are deteriorated, and the allowable communication range is thus narrowed.